1. Field of the Invention
The invention relates to a semiconductor device having metal gate and a manufacturing method thereof, and more particularly, to a semiconductor device having metal gate and a manufacturing method capable of prevention metal diffusion and improving gap-fill result.
2. Description of the Prior Art
With a trend toward scaling down the size of the semiconductor device, work function metals are used to replace the conventional polysilicon gate to be the control electrode that competent to the high dielectric constant (herein after abbreviated as high-k) gate dielectric layer. The conventional metal gate methods are categorized into the gate first process and the gate last process. Among the two main processes, the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-k gate dielectric layer and the metal gate, and thus the gate last process gradually replaces the gate first process.
In the conventional gate last process, a dummy gate or a replacement gate is formed on a substrate and followed by steps of forming a conventional metal-oxide semiconductor (MOS) transistor device. Subsequently, the dummy/replacement gate is removed to form a gate trench. Then the gate trench is filled with work function metals required by different conductivity types and gap filling metals. Often, it may employ material such as aluminum (Al) as the gap filling metal. It has been observed that Al may diffuse into the work function metals, and thus the electrical property of the work function metal is adversely influenced. As a countermeasure against to the problems, there has been proposed the barrier layers to prevent the Al diffusion.
To provide prevention to the Al diffusion, multi-layered barrier structure including at least a titanium nitride (hereinafter abbreviated as TiN) layer or a tantalum nitride (hereinafter abbreviated as TaN) is developed. However, it is found the multi-layered barrier structure is still insufficient to prevent the Al diffusion. Furthermore, the multi-layered barrier structure narrows the opening of the gate trench and thus causes gap-filling issue.
Accordingly, though the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-k gate dielectric layer and the metal gate, the gate last process still faces material requirements for the complicated processes and reliability requirement for the layers filling in the gate trench.